Automatic dishwashing (hereinafter ADW) machines employ a variety of wash cycles, or in the case of commercial practice, a variety of machine stages, which usually include a pre rinse, one or more spray washings using an aqueous detergent solution, and one or more rinses to remove residual detergent and loosened soil. In the majority of modern machines, a rinse aid composition is added, via a separate dispenser, to the final rinse cycle or stage, which composition serves to promote wetting, enhance sheet flow production and increase the rate of water drainage, thereby reducing water spotting on the washed and dried tableware. The rinse aid, which is liquid, contains a low foaming nonionic surfactant and a chelating agent in a hydrotrope-water solubilising system.
In areas where the water supply has a low level of mineral hardness i.e..ltoreq.50 ppm expressed as CaCO.sub.3, or in ADW machines whose water supply is presoftened, it has been noticed that glassware subjected to repetitive washing in an ADW machine develops a surface cloudiness which is irreversible. This cloudiness often manifests itself as an iridescent film that displays rainbow hues in light reflected from the glass surface and the glass becomes progressively more opaque with repeated treatment. Whilst the source of this cloudiness is not completely understood, it is believed that it arises from chelating agent carried over from the wash or contained in the rinse aid, attacking the glass surface during the final rinse or the subsequent drying step.
The corrosion of glass by detergents is a well known phenomenon and a paper by D. Joubert and H. Van Daele entitled "Etching of glassware in mechanical dishwashing" in Soap and Chemical Specialities, March 1971 pp62, 64 and 67 discusses the influence of various detergent components particularly those of an alkaline nature. Zinc salts incorporated as components of the detergent compositions are stated to have an inhibitory effect on their corrosive behaviour towards glass.
This subject is also discussed in a paper entitled "The present position of investigations into the behaviour of glass during mechanical dishwashing" presented by Th. Altenschoepfer in April 1971 at a symposium in Charleroi, Belgium on "The effect of detergents on glassware in domestic dishwashers". In the paper the use of zinc ions in the detergent compositions used to wash glsss was stated to provide too low a "preservation factor". A similar view was also expressed in another paper delivered at the same symposium by P. Mayaux entitled "Mechanism of glass attack by Chemical Agents".
Rutkowski U.S. Pat. No. 3,677,820 discloses the use of metallic zinc or magnesium strips in automatic dishwashing machines to inhibit glassware corrosion caused by the alkaline detergent solution, and the incorporation of calcium, beryllium, zinc and aluminum salts into ADW detergent compositions for the same purpose is disclosed in U.S. Pat. Nos. 2,447,297 and 2,514,304, German DTOS No. 2,539,531 and B.P. No. 1,517,029. None of the above references discuss the corrosion of glass arising from treatment with a solution of a chelating agent in water of low mineral hardness and close to neutral pH, such as takes place when a conventionally formulated rinse aid is added to the final rinse stage of an ADW machine cycle. It has surprisingly been found that the addition of water soluble Zn or magnesium salts to the final rinse substantially eliminates this soft water corrosion.